A transparent electroconductive film having a high conductivity and a high transmittance in a visible light range has been used in a solar cell, a liquid crystal display element and an electrode and the like for various light receiving elements, and in addition to these, has also been used as a heat-reflective film for car windows and for construction purposes, an antistatic film and various kinds of transparent antifogging heat generators for use in refrigerated display cases and the like.
As the transparent electroconductive film, tin oxide (SnO2)-based, zinc oxide (ZnO)-based and indium oxide (In2O3)-based thin films have been utilized. As the tin oxide-based film, those containing antimony as a dopant (ATO) and those containing fluorine as a dopant (FTO) have been utilized. As the zinc oxide-based film, those containing aluminum as a dopant (AZO) and those containing gallium as a dopant (GZO) have been utilized. The transparent electroconductive film that has been industrially used most widely is the indium oxide-based film, and among these, the film made of indium oxide containing tin as a dopant is referred to as ITO (Indium-Tin-Oxide) film, and this has been widely utilized because this makes it possible to easily provide, in particular, a film having a low resistivity.
In recent years, attentions have been focused on a problem with the earth environment due to an increase of carbon dioxide and the like and a problem of sudden price hikes of fossil fuels, and thin-film solar cells that can be produced at comparatively low costs have drawn public attentions. In general, the thin-film solar cell includes a transparent electroconductive film and one or more semiconductor thin-film photoelectric conversion units that are successively stacked on a translucent substrate and back electrodes. Among thin-film solar cells, since silicon materials are rich in resources, a silicon-based thin-film solar cell in which a silicon-based thin-film is used as a photoelectric conversion unit (light absorbing layer) has been quickly put into practical use, and its researches and developments have been vigorously carried out.
Moreover, various kinds of silicon-based thin-film solar cells have been introduced, and in addition to an amorphous thin-film solar cell in which an amorphous thin-film made of amorphous silicon or the like is used as a conventional light absorbing layer, a microcrystalline thin-film solar cell which uses a microcrystalline thin-film, with microcrystalline silicon being mixed in amorphous silicon, and a crystalline thin-film solar cell in which a crystalline thin film made of crystalline silicon is used have been developed, and a hybrid thin-film solar cell in which these are stacked has also been put into practical use.
With respect to the photoelectric conversion unit or the thin-film solar cell, regardless of whether the p-type and n-type conductive semiconductor layers included therein are amorphous, crystalline or microcrystalline, the unit in which a photoelectric conversion layer that occupies its main portion is made of amorphous is referred to as an amorphous unit or an amorphous thin-film solar cell, the unit in which a photoelectric conversion layer is made of crystalline is referred to as a crystalline unit or a crystalline thin-film solar cell, and the unit in which a photoelectric conversion layer is made of microcrystalline is referred to as a microcrystalline unit or a microcrystalline thin-film solar cell.
By the way, the transparent electroconductive film is used for surface transparent electrodes of a thin-film solar cell, and normally a large number of fine unevenness are formed on its surface so as to effectively confine light that is made incident on the translucent substrate side inside the photoelectric conversion unit.
As an index indicating the degree of unevenness of this transparent electroconductive film, a haze ratio is used. This corresponds to a rate obtained by dividing diffused components whose light paths are bent of light rays that are allowed to transmit the translucent substrate with the transparent electroconductive film when light from a specific light source is made incident thereon, by the entire components thereof, and is measured by using a C-light source including normal visible light. In general, as the height difference of the unevenness is increased, or as an interval between the convex portions of the unevenness is made greater, the haze ratio becomes higher, and light made incident on the photoelectric conversion unit is effectively confined so that a so-called light confinement effect becomes superior.
Regardless of whether the thin-film solar cell is a thin-film solar cell having a single-layer light absorbing layer made of amorphous silicon, crystalline silicon or microcrystalline silicon, or the above-mentioned hybrid thin-film solar cell, a high short-circuit electric current density (Jsc) can be achieved as long as a sufficient light confinement process can be carried out by increasing the haze ratio of the transparent electroconductive film, so that a thin-film solar cell having a high conversion efficiency can be manufactured.
Based upon the above-mentioned purposes, a metal oxide material, which is mainly made of tin oxide, and produced by a thermal CVD method, has been known as the transparent electroconductive film having a high degree of unevenness and a subsequent high haze ratio, and has been generally used as a transparent electrode for a thin-film solar cell.
In general, the conductivity-type semiconductor layer to be formed on the surface of a transparent electroconductive film is manufactured in a gaseous atmosphere containing hydrogen by using a plasma CVD method. In the case when a formation temperature is made higher so as to allow the conductivity-type semiconductor layer to contain microcrystal, a reducing process of metal oxide is accelerated by hydrogen that is present, with the result that in the case of a transparent electroconductive film mainly composed of tin oxide, the loss of transparency due to hydrogen reduction appears. In the case of using such a transparent electroconductive film in which transparency deteriorates, it is not possible to achieve a thin-film solar cell having high conversion efficiency.
As a method for preventing hydrogen reduction in the transparent electroconductive film mainly composed of tin oxide, non-patent document 1 has proposed a method in which, on a transparent electroconductive film that is made from tin oxide having a high degree of unevenness, and formed by a thermal CVD method, a zinc oxide film having a superior reduction resistance is thinly formed by using a sputtering method. It is disclosed that since zinc oxide has a strong bond between zinc and oxygen and exerts superior hydrogen-resistant reducibility, it is possible to maintain high transparency in the transparent electroconductive film by using the above-mentioned structure.
However, in order to obtain a transparent electroconductive film having the above-mentioned structure, a film-forming process has to be carried out by combining two kinds of methods with each other, with the result that great costs are required, failing to provide a practical method. Moreover, with respect to the method in which all the tin oxide-based transparent electroconductive film and the zinc oxide-based transparent electroconductive film are manufactured by using a sputtering method, due to reasons that a tin oxide-based transparent electroconductive film having high transparency cannot be produced by the sputtering method and the like, it is considered that the method cannot be achieved.
Non-patent document 2 has proposed a method in which a transparent electroconductive film that is mainly composed of zinc oxide and has surface unevenness and a high haze ratio is obtained by using a sputtering method. In this method, by using a sintered body target of zinc oxide to which 2 wt % of Al2O3 has been added, a sputtering film-forming process is carried out under a high gas pressure of 3 to 12 Pa, with its substrate temperature being set to 200 to 400° C. However, since the film-forming process is carried out by applying a power of DC 80 W to a target with 6 inches φ, the applied power density to the target is 0.442 W/cm2, which is a very low level. For this reason, the deposition rate is extremely slow, that is, 14 to 35 nm/min, failing to provide a practical method from the industrial viewpoint.
Moreover, non-patent document 3 has disclosed a method in which, after obtaining a transparent electroconductive film mainly composed of zinc oxide and having small surface unevenness by using a conventional sputtering method, the surface of the film is etched by acid so as to form surface unevenness, thereby manufacturing a transparent electroconductive film with a high haze ratio. In this method, however, since, after a film has been produced through a dry process, by using a sputtering method that is a vacuum process, an acid etching process is carried out thereon in the air, and then dried, and since this has to be again formed into a semiconductor layer by using a CVD method that is a dry process; therefore, complicated processes are required to cause a problem of high costs or the like.
With respect to AZO containing aluminum as a dopant among zinc oxide-based transparent electroconductive film materials, a method has been proposed in which by using a target mainly composed of zinc oxide, with aluminum oxide being mixed therein, an AZO transparent electroconductive film that is C-axis oriented is manufactured by a DC magnetron spattering method (see Patent Document 1). In this case, when a DC sputtering film-forming process is carried out by increasing a power density to be applied to a target so as to form a film at high speeds, arching (abnormal discharge) tends to occur frequently. When arching frequently occurs in the manufacturing process in the film-forming line, defects tend to occur in the film, and a film having a predetermined film thickness cannot be obtained, with the result that it becomes impossible to manufacture a transparent electroconductive film having high quality in a stable manner.
For this reason, the present applicant has proposed a sputtering target, mainly composed of zinc oxide, with gallium oxide being mixed therein, to which by adding a third element (Ti, Ge, Al, Mg, In, Sn), an abnormal discharge is reduced (see Patent Document 2). In this case, a GZO sintered body containing gallium as a dopant is composed of a ZnO phase as its main phase in which 2 wt % or more of at least one member selected from the group consisting of Ga, Ti, Ge, Al, Mg, In and Sn is contained as a solid solution, with other constituent phases being a ZnO phase in which at least one member selected from the above group is not contained as a solid solution and an intermediate compound phase represented by ZnGa2O4 (spinel phase). Although this GZO target to which the third element such as Al is added makes it possible to reduce an abnormal discharge, as described in Patent Document 1, but cannot completely eliminate the abnormal discharge. In the continuous line of the film-forming process, if an abnormal discharge occurs even once, the resulting film product becomes a defective product, causing an adverse effect to the production yield.
In order to solve this problem, the present applicant has formed an oxide sintered body mainly composed of zinc oxide in which aluminum and gallium are further contained as additive elements, and by optimizing the contents of aluminum and gallium, as well as by controlling the kind and composition, in particular, the composition of the spinel crystal phase, of the crystal phase generated during the sintering process, has also proposed a target-use oxide sintered body that hardly causes particles even when a film-forming process is carried out in a sputtering apparatus for a long period of time, with no abnormal discharge being caused even upon application of a high DC power thereto (see Patent Document 3). By using this target-use oxide sintered body, it becomes possible to form a transparent electroconductive film with high quality, having a lower resistivity and a high transmittance in comparison with the conventional film, and consequently to apply this method to a manufacturing process for a solar cell with high conversion efficiency.
However, in recent years, there have been strong demands for solar cells having a higher conversion efficiency, and transparent electroconductive films having high quality have been required.